Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Various radiation-sensitive compositions that can be used to generate free radicals upon thermal imaging and imageable elements containing same are described in numerous publications such as U.S. Pat. No. 6,309,792 (Hauck et al.) describing negative-working elements that include polycarboxylic acids. Other IR-sensitive negative-working elements are described in U.S. Patent Application 2009/0111051 (Tao et al.) and WO 2004/041544 (Munnelly et al.).
In many instances, such negative-working imageable elements require a topcoat over the imageable layer to act as an oxygen barrier to provide desired shelf life and sensitivity, but there is a desire to eliminate this extra layer because of material costs and the need for additional coating equipment. The presence of the topcoat often requires an extra washing step before the imaged layer is developed in a suitable developer or it will reduce the developer life in a processing cycle of an automatic processor. The elimination of the topcoat, however, negatively impacts the sensitivity and shelf-life (stability over time) of the printing plate precursor. It is desirable to eliminate this oxygen barrier topcoat without any loss in sensitivity and shelf-life stability. Moreover, other imageable elements require a baking step between imaging and development (processing) in order to increase image wearability, but it would also be desirable to eliminate this extra step for the user (customer) without any loss in imaging speed (imaging sensitivity).